Vacuum pumps and aspirators are common pieces of equipment in analytical, chemical synthesis and clinical laboratories, where they are useful for solvent removal from reaction product, vacuum filtration, and similar small-scale tasks. Biochemical laboratories utilize vacuum in a variety of procedures, one of the most common being the drying of gels in which any of various different types of electrophoresis has been performed. Other procedures and equipment with which vacuum is used include freeze drying, rotary evaporators, vacuum concentrators, distillation apparatus, filtering flasks, degassing equipment, desiccation, fume and vapor removal, vacuum dialysis, and vacuum ovens.
Pumps which draw vacuum directly include a vapor trap to protect the pump from corrosive vapors which might damage the pump. The oil used in these pumps must be periodically drained from both the vapor trap and the pump itself, and the pumps still entail a risk of drawing too great a vacuum and mining the experiment. There are also the risks of drawing destructive materials into the pump, requiring costly repairs or replacement of the pump itself, and of expelling oil from the pump into the surrounding air. Other disadvantages are the cost of the oil and the problem of disposal of used oil. The use of these pumps to dry gels further presents the risk of releasing acetic acid into the atmosphere, since acetic acid is entrained with the water drawn from the gel by the vacuum.
Aspirators, or water jet pumps, are widely used in place of vacuum pumps, since aspirators avoid many of the dangers and operating costs associated with oil-based pumping systems. Aspirators are particularly useful for drying gels, since the water in an aspirator serves as an effective trap for the acetic acid. The simplest aspirators are those that are connected directly to a tap water line, where one can simply turn on the tap to start the vacuum. These aspirators are not reliable, of course, at locations where water pressure is low or unsteady. The greatest disadvantage, however, is the high consumption of water. Operators often forget that the tap is running or are too preoccupied to turn it off, leaving it on for hours and wasting precious tap water.
To avoid wastage of water, self-contained aspirator vacuum systems are currently marketed. The typical system contains a water tank with a motor-driven circulating pump immersed in the tank. The pump draws water from the tank and forces the water through one or more aspirators that are part of the system itself, before returning the water to the tank. A disadvantage of these systems is that the metallic pump shaft is immersed in the water and readily conducts the heat generated by the pump to the water, causing the water temperature to rise. This causes the flow rate to drop, which in turn results in a weaker vacuum. A large pump motor is often used in an attempt to compensate for this, but the result is a faster rise in temperature.